This invention relates to an electrical brush assembly for use with a d.c. dynamoelectric machine commutator having a substantially cylindrical commutator contact surface.
FIG. 1 illustrates one example of a conventional electrical brush assembly 10 for use with a d.c. dynamoelectric machine including a commutator 12 having a substantially cylindrical commutator contact surface 14 in which a plurality of commutator segments (not shown) are formed. The outer diameter of the cylindrical commutor 12 may be from about 30 mm to about 40 mm. The brush assembly 10 comprises an electrical brush element 16 which is a rod-shaped electrically conductive member having a substantially flat end surface 18 constituting a sliding contact to be brought into contact with contact surface 14 of the commutator 12. The brush element 16 also has side support surfaces 20 which are to be slidably supported. The width dimension W of the brush element 16 as measured in the direction of rotation of the commutaor contact surface 14 may typically be from 4 mm to 7 mm.
The brush assembly 10 further comprises a brush holder 22 for slidably supporting the side support surfaces 20 of the brush element 16 for a sliding movement relative to the commutator 12 in a diametrical direction of the commutator and such that the contact surface 18 of the brush element 16 can be in a sliding contact relationship with the contact surface 14 of the commutator 12. In order to ensure a smooth sliding movement between the brush holder 22 and the brush element 16 supported therein, a small gap 23 of from 0.05 mm to 0.30 mm is defined between the inner surface of the brush holder 22 and the side support surface 20 of the brush element 16. This gap 23 is also necessary for easy assembly of the brush element 16 into the holder 22. The brush assembly 10 includes a compression spring 24 for urging the sliding contact surface 18 of the brush element 16 against the contact surface 14 of the commutator 12.
The brush element 16 has a central axis 26 which extends in a substantially radial direction with respect to the commutator 12 and which is perpendicular to the central axis 28 of the cylindrical commutator contact surface 14. Therefore, the sliding contacting surface 18 of the brush element 16 contacts with the contact surface 14 of the commutator 12 only at a line 30 between the brush element 16 and the commutator 12.
With the structure as above described, since the contact surface 18 of the brush element 16 is flat and the commutator contact surface 14 is cylindrical, the brush element 16 and the commutator 12 are in a line contact extending parallel to the central axis 28 as indicated by a line 30, making the brush unstable. Therefore, the brush element 16 chatters and bounces on the rotating coummutator 12, resulting in a poor electrical contact between the brush element 16 and the commutator 12 as well as the generation of a large noise. This lasts for a certain period of time at the initial stage of using a fresh brush element 16.
As the commutator 12 continues to rotate, the contact surface 18 of the brush element 16 wears as it is urged by the compression spring 24 and finally the entire contact surface 18 of the brush element 16 becomes a concave cylindrical surface having the same radius of curvature as that of the commutator contacting surface 14. At this stage, the brush element 16 is stable and no noise is generated and the desired good electrical contact is ensured between the brush element 16 and the commutator 12.
Therefore, an electrical brush assembly 32 as shown in FIG. 2 has been proposed in which a brush element 34 manufactured to have a sliding contact surface 36 that is a concave cylindrical surface is used from the very begining of operation of the machine. The concave cylindrical surface has a central axis 38 that coincides with the central axis 28 of the commutator contact surface 14 and therefore has a radius equal to the radius of the commutator contact surface 14, thereby establishing a surface contact between the commutator contact surface 14 and the entire area of the brush element contact surface 36.
With this arrangement, however, since there is provided a small gap 23 between the side support surface 20 of the brush element 34 and the inner surface of the brush holder 22, the brush element 16 is often diagonally assembled into the brush holder 22 as shown in FIG. 3. In this position, the sliding contact surface 36 of the brush element 34 is brought into contact only at one corner 40 of the element 34. This means that the position of contact of the brush element 34 to the commutator 12 is significantly shifted resulting in an effective change of the brush angle, causing the initial characteristics of the brush assembly to be very unstable.